DRI, which is sometimes referred to as sponge iron, is typically produced by the reaction between iron ore and a reactive gas stream containing reducing agents like H2 and CO. In commercialized DR processes, a hydrocarbon source is normally utilized to produce the reducing agents via a catalytic (e.g., tubular reformer) or non-catalytic (e.g., partial oxidation (PDX)) reforming process. All of these reforming processes convert some portion of the hydrogen and carbon content of the hydrocarbons into H2 and CO, respectively.
In conventional DR processes, a hot H2/CO-rich stream (referred to as a reducing gas stream) flows into a reduction reactor, typically a vertical shaft reactor, and reacts with iron oxides based on the following global reactions:Fe2O3+3H22Fe+3H2O  (1)Fe2O3+3CO2Fe+3CO2  (2)
Although the above simplified reaction scheme does not describe much about the details of the gas-solid reactions occurring during the reduction of iron ores with H2 and CO, it adequately shows the overall fate of the reducing agents, H2 and CO, inside the reduction reactor, i.e., they are converted to H2O and CO2, respectively.
In order to prevent reverse reforming reactions (i.e., methanation reactions) inside the reduction reactor, some CH4 in the form of natural gas (NG) or the like is typically added to the reducing gas stream before flowing into the reduction reactor.
Due to the equilibrium-limited nature of reduction reactions under practical operating conditions, the presence of reduction reaction products, H2O and CO2 (i.e., oxidants), in the reacting mixture inhibits the complete utilization of the H2 and CO supplied to the reduction reactor. Therefore, top gas that is the spent gas coming off the reduction reactor still contains considerable amounts of the reducing agents, and it is worth removing the oxidants and reusing unreacted H2 and CO for further reduction. In commercialized DR processes, the removal of H2O is usually carried out by quenching the top gas down to ambient temperature to condense out the majority of its moisture content. However, the removal of CO2 is not so straightforward, and typically requires the installation of a separate process to capture the CO2 from the top gas. Although there are many successfully commercialized processes for this purpose, all of them require significant capital investments and have high operating costs.
The product DRI can be then used as a good source of low-residual iron, in addition to pig iron and ferrous scrap in the production of steel, mainly through an electric arc furnace (EAF) in a steelmaking facility. The EAF melts the charged material by means of an electric arc. The presence of carbon in the DRI loaded into EAF is equivalent to adding chemical energy to the EAF when oxygen is injected into the EAF. Partial and complete combustion of the carbon with oxygen provides a uniform internal source of energy within the EAF charge. Furthermore, the conversion of Fe3C into iron and carbon is an exothermic reaction, which improves the thermal efficiency of the EAF as well. Therefore, the carbon content of the DRI can be interpreted as an energy source, and this energy is finally utilized in the EAF when the DRI is melted. Although other carbon sources, such as coal or used rubber, can be added to the EAF for the same purpose, the resulting yield is significantly less than the combined carbon in DRI due to particle blow-off and impurities existing in these carbon sources.
Inside the reduction reactor, carbon can be generated (i.e., physical carbon-C) or added to the DRI (i.e., chemical carbon-Fe3C) mainly through the following global reactions:3Fe+CO+H2Fe3C+H2O  (3)3Fe+2COFe3C+CO2  (4)3Fe+CH4Fe3C+2H2  (5)CO+H2C+H2O  (6)2COC+CO2  (7)CH4C+2H2  (8)
Therefore, two major sources for combined carbon (i.e., chemical+physical) are CO and hydrocarbons (e.g., CH4) in the reducing gas stream.
One widespread source of hydrocarbons in the iron and steel industry is COG, which typically contains 20.0%-28% methane. Due to this considerable CH4 concentration, COG can be reformed into H2 and CO in order to reduce iron oxide to metallic iron, in the form of DRI, hot direct reduced iron (HDRI), or hot briquetted iron (HBI) in a direct reduction plant. A typical COG stream coming from a COG treatment plant also contains between 50.0%-65.0% H2, 4.0%-8.0% CO, up to 2.0% aromatics (typically in the form of BTX), and up to 5.0% of higher hydrocarbons like ethane, propane, and some kinds of olefins. Because of the high concentration of H2 in a COG stream, reforming COG typically results in a Syngas with significantly higher H2/CO, as compared to reforming natural gas. In other words, the amount of carbon introduced into the shaft furnace in the form of CO is less in the case of COG reforming.
The presence of such high concentrations of H2 in COG also has adverse consequences for both catalytic and non-catalytic reforming processes since it is the main product of reforming reactions; and, therefore, reduces the efficiency of the reforming reactions. In other words, since the rate of reforming reactions is slower in the presence of high concentrations of hydrogen, more energy is consumed to reform the hydrocarbons to H2 and CO. At the same time, although a typical COG stream can contain up to 65.0% of reducing agents CO and H2, with no reforming step, the CH4 content of the COG will accumulate within the system if the COG flows directly into the DR process loop.
In addition, the presence of heavy hydrocarbons, such as olefins and aromatics, along with very heavy hydrocarbons, such as tar and naphthalene (typically in the form of liquid carry-over), as well as sulfur compounds (typically more than 100 ppm), in the COG makes it difficult to reform the COG in conventional catalytic processes, as these components deactivate the commercial reforming catalysts relatively easily. Accordingly, expensive cleaning processes are typically required to remove these components from the COG upstream of the catalytic reformers, which in turn makes the whole process extremely costly, as compared to other non-catalytic methods, such as PDX.
Thus, the present “state of the art” describes an economic DR process for the production of high carbon content DRI when COG is the available source of hydrocarbons for the plant, in which top gas divides into two different streams. One portion mixes with COG and flows into a selective separation unit, such as a pressure swing adsorption (PSA) system with solid adsorbent or the like, for the adjustment of CO2 and CH4 content, while the other portion of the gas bypasses the separation unit. The product gas from the separation unit then blends with the bypassed stream before flowing to a heater. The resulting hot gas coming from the heater contains a high amount of CH4 (preferably more than 6.0%) and a low amount of CO2 (preferably less than 3.0%). Due to this high CH4 content, the product DRI contains a high combined carbon content before leaving the hot section of the reduction furnace based on the following reactions:3Fe+CH4Fe3C+2H2  (5)CH4C+2H2  (8)
The proposed design does not include any reforming step; and, therefore, is cost effective and simple in both operation and maintenance. In fact, in case of using solid adsorbent materials in a PSA system, the separation unit acts as a N2 and CH4 concentration adjustor, performs CO2 removal, performs sulfur removal, and acts as a fuel generating unit at the same time. Consequently, almost all of the top gas can be recycled to the process loop for boosting the plant capacity.
The flow sheet of the present invention also offers outstanding performance in terms of oxygen consumption for producing DRI. While a PDX-based design requires more than 60 Nm3 of oxygen per ton of produced metallic iron, the proposed design needs less than 30 Nm3 of oxygen per ton of produced metallic iron, when COG is used as the main external source of reducing agents in the plant.
Finally, the present invention makes use of industrially well-practiced technologies for separation purposes. Numerous selective separation technologies units have been installed all around the world by different vendors for the selective separation of components from gas streams; and, thus, more than adequate industrial experience exists for this application.